Verify that the next sub-leaf of CPUID 0x4 (or 0x8000001d) is validbefore rejecting the entire KVM_GET_SUPPORTED_CPUID due to insufficentspace in the userspace array.

Note, although this is technically a bug, it's not visible to userspaceas KVM_GET_SUPPORTED_CPUID is guaranteed to fail on KVM_CPUID_SIGNATURE,which is hardcoded to be added after the affected leafs. The realmotivation for the change is to tightly couple the nent/maxnent anddo_host_cpuid() sequences in preparation for future cleanup.